1. Field of the Invention
The invention relates generally to medical instruments for examining the anatomy of the eye, and more particularly to an improved apparatus and method for visualizing translucent or transparent internal anatomical structures at the back of the eye.
2. Description of the Related Art
In-vivo examination of the retina of the eye is routinely performed during ophthalmological examinations because small changes in the condition of the retina are often indicative of the onset of several sight threatening diseases, such as glaucoma, diabetes, cystoid macular edema, age related macular degeneration, subretinal neovascular membranes, retinal edema arising as a result of various vascular diseases and diseases of the cranial nerves. For example, detectable changes in the condition of the optic nerve head may reflect the loss of retinal ganglion cell axons, which has been found to occur in the early stages of glaucoma. Similarly, changes in the neuroretinal rim and the optic disk may also be correlated with changes in vision during the course of an ocular disorder.
Degenerative diseases of the retina, or changes in the retina due to glaucoma, may be quantitatively evaluated using various methods, such as tonometry, visual field analysis and other techniques. Photographic techniques, coupled with computer enhancement of the resulting photographs may also be used to document the condition of the retina. While these methods provide valuable information about the gross anatomy and general condition of the retina, they lack the sensitivity or resolution necessary to resolve the fine structure of the retina, such as the nerve fiber layer. The ability to view the nerve fiber layer, and to be able to detect and measure changes in its structure and thickness, is important in diagnosing the onset and course of glaucoma.
Because glaucoma affects a large number of people and produces serious visual loss, a wide variety of diagnostic equipment and techniques have been utilized to view the nerve fiber layer of the retina. Most of the techniques currently used to evaluate the retina utilize a device known as the ophthalmoscope to observe the retina. Using the ophthalmoscope, a physician visualizes the retina, and, using a camera or optoelectronic imaging tube, records an image of the retina. Where the image is electronically obtained, the electronic signals comprising the image may then be processed using a computer and image enhancing software to enhance the structural details of the retina that are present.
In indirect ophthalmoscopy, examination of the retina of the eye is performed by observing a real aerial image of the retina. Various optical systems have been developed to generate the real aerial image of the retina. Typically, an ophthalmoscope consists of a light source and a combination of optical elements adapted to transmit light from the light source onto the retina and then to transmit any light scattered by the structures of the eye back to the observer for viewing and recording. For example, one system uses an objective lens to converge light from an ophthalmoscope light source onto the retina and also to form the aerial image. One problem with all of the techniques employing the ophthalmoscope is that only retinal and ocular structures capable of scattering light back into the optical elements of the ophthalmoscope are visible. Transparent or translucent structures of the eye, such as the nerve fiber layer, scatter very little of the light provided by the ophthalmoscope's light source, and so are extremely difficult to visualize.
One approach used to increase the sensitivity of the ophthalmoscope has been to increase the amount of light scattered by translucent structures such as the nerve fiber layer by increasing the amount of the light supplied by the light source. The disadvantage of this approach is that the retina is extremely sensitive to light, and can suffer permanent damage if the light provided by the light source is too intense. Thus, the maximum intensity of the light supplied by the light source of the ophthalmoscope is limited by the requirement that no damage be caused to the eye during the diagnostic procedure. Unfortunately, the amount of light scattered by the nerve fiber layer, even with the light source providing light at its maximum safe intensity, is often too small to allow visualization of subtle changes in the nerve fiber layer that are valuable in diagnosing glaucoma. Moreover, the increased intensity of light provided to the eye also results in increased scattering of the light by other structures of the eye that further obscures the detail of the nerve fiber layer by overwhelming the small percentage of the scattered light that is actually reflected by the nerve fiber layer.
Several methods have been proposed to improve nerve fiber layer visibility by increasing the relative amount of light scattered by the nerve fiber layer compared to other ocular structures. One approach has been to illuminate the retina with light having a wavelength that is preferentially scattered by the nerve fiber layer and which is not scattered by other ocular structures. One such method interposed a green filter between the light source of the ophthalmoscope and the eye, but has been unsuccessful in obtaining images with enough resolution to view the nerve fiber layer because there is only a small difference in the scattering of light between the nerve fiber layer and other ocular structures at the wavelength of light transmitted by the green filter.
Another approach utilized polarized light in an attempt to increase the relative scatter of the light by the nerve fiber layer based on the theory that the nerve fibers, being long thin cylinders, should be strongly birefringent. This approach was also unsuccessful in significantly improving the visualization of the nerve fiber layer because the nerve fiber layer proved to be only weakly birefringent; thus the expected increase in scattered light by the nerve fiber layer in relation to non-birefringent structures of the eye was not obtained.
Alternative approaches that do not directly visualize individual nerve fibers but instead measure some anatomical feature of the nerve fiber layer have also been attempted. Ellipsometry, which can be used to estimate the thickness of the nerve fiber layer by measuring the degree of birefringence of the nerve fiber layer, is one such method. Using ellipsometry, it is possible to determine the birefringence of the nerve fiber layer at several points on the surface of the retina. The thickness of the nerve fiber layer may then be calculated from these measurements since the degree of birefringence is dependent upon the thickness of the layer. However, the resolution of this technique is limited because the other structures of the eye are significantly more birefringent than the nerve fiber layer; light scattered by these structures interferes with the measurement and reduces its accuracy. Thus, this method can only measure relatively large differences in the thickness of the nerve layer, and is incapable of detecting the subtle changes in nerve fiber layer thickness caused by glaucoma.
Several methods have been used to produce cross-sectional images of the retina in an attempt to detect changes in the nerve fiber layer. One such method is confocal scanning laser ophthalmoscopy. While this method is capable of producing cross-sectional images of the nerve fiber layer, the images produced do not have sufficient resolution to enable detection of small changes in the thickness of the nerve layer. Another method capable of producing cross-sectional views of the retina is coherence domain tomography, also known as laser doppler interferometry. In theory, this method should be capable of estimating the thickness of the nerve fiber layer with an accuracy of ten microns, but is adversely affected by the amount of light scattered intraretinally by the nerve fiber layer. As a practical matter, this method is useful for detecting retinal defects extending through the full thickness of the retina, but has not provided the accuracy needed to monitor changes in the thickness of the nerve fiber layer caused by glaucoma. In addition to the specific disadvantages discussed above, ellipsometry, confocal laser ophthalmoscopy and coherence domain tomography are all incapable of visualizing individual nerve fibers in the nerve fiber layer.
When an image of the retina has been obtained and stored using optoelectronic means, as discussed above, or where a photograph of an image has been digitized, the image may be processed using specialized computer software to enhance the details of the image. The degree to which a stored image can be enhanced, however, depends on the quality and detail of the original image. Where the original image contains little information, as in the case where little light is scattered by the nerve fiber layer, image processing is typically of little value.
What has been needed, and heretofore unavailable, is an accurate and reliable apparatus and method for visualizing individual nerve fibers in the retina and for measuring the thickness of the nerve fiber layer to a high degree of accuracy without requiring unsafe or uncomfortable levels of illumination.